A large body of evidence points to the importance of endogenous opioids in alcohol drinking and reinforcement (1). The medial shell of nucleus accumbens (NAc) has emerged as an important neural substrate for these effects and may mediate problem alcohol use (2-9). Importantly, infusion of the mu-opioid receptor agonist, DAMGO, into rodent NAc potentiates alcohol consumption (10), yet the behavioral and neurological mechanisms of this effect remain unknown. Medial NAc shell is a critical relay for prefrontal cortical influences on both hypothalamus and ventral pallidum, forming an extinction circuit which may co -opt mechanisms normally devoted to satiety, and modulation of this circuit may serve as a critical mechanism of opioid-induced alcohol consumption and relapse (11-12). Glutamatergic inputs from infralimbic cortex to medial NAc shell are posited to suppress drug and alcohol seeking following extinction and during reinstatement (13- 14), potentially via inhibitory projections from NAc to ventral pallidum and lateral hypothalamus (11, 15). The proposed experiments will systemically test the psychological and neural mechanisms of NAc DAMGO- induced alcohol consumption, and in particular the effects of NAc shell DAMGO on this extinction circuit. The experiments in Aim 1 of this proposal will test the specific effects o NAc shell DAMGO in alcohol self - administration and cue-induced relapse to alcohol seeking, to determine what aspects of alcohol reward are enhanced by NAc mu-opioid signaling (i.e. palatability, motivation, or reinforcement). It is critical to understand how NAc shell mu-opioid signaling affects top-down inputs from infralimbic cortex that may normally suppress alcohol seeking and relapse. Aim 2 of the research plan will test whether selective activation of infralimbic inputs to NAc can suppress alcohol consumption and relapse, and whether NAc shell DAMGO blocks these effects, during alcohol self-administration and relapse. It is also vital to understand how NAc mu-opioid induced changes in alcohol seeking are encoded in downstream structures of the extinction circuit, including ventral pallidum and lateral hypothalamus, which may encode different aspects of alcohol reward. The experiments in Aim 3 will test whether neural activity in lateral hypothalamus or ventral pallidum encodes alcohol seeking, consumption, or relapse to alcohol seeking, or enhancements in any of these processes following NAc shell DAMGO.